(* The term articulation is used in this chapter to denote both
"segmentation" and "articulation" in the ordinary sense.--Translator.)
The vertebrate stem, to which our race belongs as one of the latest
and most advanced outcomes of the natural development of life, is
rightly placed at the head of the animal kingdom. This privilege must
be accorded to it, not only because man does in point of fact soar far
above all other animals, and has been lifted to the position of "lord
of creation"; but also because the vertebrate organism far surpasses
all the other animal-stems in size, in complexity of structure, and in
the advanced character of its functions. From the point of view of
both anatomy and physiology, the vertebrate stem outstrips all the
other, or invertebrate, animals.
There is only one among the twelve stems of the animal kingdom that
can in many respects be compared with the vertebrates, and reaches an
equal, if not a greater, importance in many points. This is the stem
of the articulates, composed of three classes: 1, the annelids
(earth-worms, leeches, and cognate forms); 2, the crustacea (crabs,
etc.); 3, the tracheata (spiders, insects, etc.). The stem of the
articulates is superior not only to the vertebrates, but to all other
animal-stems, in variety of forms, number of species, elaborateness of
individuals, and general importance in the economy of nature.
When we have thus declared the vertebrates and the articulates to be
the most important and most advanced of the twelve stems of the animal
kingdom, the question arises whether this special position is accorded
to them on the ground of a peculiarity of organisation that is common
to the two. The answer is that this is really the case; it is their
segmental or transverse articulation, which we may briefly call
metamerism. In all the vertebrates and articulates the developed
individual consists of a series of successive members (segments or
metamera = "parts"); in the embryo these are called primitive segments
or somites. In each of these segments we have a certain group of
organs reproduced in the same arrangement, so that we may regard each
segment as an individual unity, or a special "individual" subordinated
to the entire personality.
The similarity of their segmentation, and the consequent physiological
advance in the two stems of the vertebrates and articulates, has led
to the assumption of a direct affinity between them, and an attempt to
derive the former directly from the latter. The annelids were supposed
to be the direct ancestors, not only of the crustacea and tracheata,
but also of the vertebrates. We shall see later (Chapter 2.20) that
this annelid theory of the vertebrates is entirely wrong, and ignores
the most important differences in the organisation of the two stems.
The internal articulation of the vertebrates is just as profoundly
different from the external metamerism of the articulates as are their
skeletal structure, nervous system, vascular system, and so on. The
articulation has been developed in a totally different way in the two
stems. The unarticulated chordula (Figures 1.83 to 1.86), which we
have recognised as one of the chief palingenetic embryonic forms of
the vertebrate group, and from which we have inferred the existence of
a corresponding ancestral form for all the vertebrates and tunicates,
is quite unthinkable as the stem-form of the articulates.
All articulated animals came originally from unarticulated ones. This
phylogenetic principle is as firmly established as the ontogenetic
fact that every articulated animal-form develops from an unarticulated
embryo. But the organisation of the embryo is totally different in the
two stems. The chordula-embryo of all the vertebrates is characterised
by the dorsal medullary tube, the neurenteric canal, which passes at
the primitive mouth into the alimentary canal, and the axial chorda
between the two. None of the articulates, either annelids or
arthropods (crustacea and tracheata), show any trace of this type of
organisation. Moreover, the development of the chief systems of organs
proceeds in the opposite way in the two stems. Hence the segmentation
must have arisen independently in each. This is not at all surprising;
we find analogous cases in the stalk-articulation of the higher plants
and in several groups of other animal stems.
The characteristic internal articulation of the vertebrates and its
importance in the organisation of the stem are best seen in the study
of the skeleton. Its chief and central part, the cartilaginous or bony
vertebral column, affords an obvious instance of vertebrate
metamerism; it consists of a series of cartilaginous or bony pieces,
which have long been known as vertebrae (or spondyli). Each vertebra
is directly connected with a special section of the muscular system,
the nervous system, the vascular system, etc. Thus most of the "animal
organs" take part in this vertebration. But we saw, when we were
considering our own vertebrate character (in Chapter 1.11), that the
same internal articulation is also found in the lowest primitive
vertebrates, the acrania, although here the whole skeleton consists
merely of the simple chorda, and is not at all articulated. Hence the
articulation does not proceed primarily from the skeleton, but from
the muscular system, and is clearly determined by the more advanced
swimming-movements of the primitive chordonia-ancestors.
(FIGURES 1.153 TO 1.155. Sole-shaped embryonic disk of the chick, in
three successive stages of development, looked at from the dorsal
surface, magnified about twenty times, somewhat diagrammatic. Figure
1.153 with six pairs of somites. Brain a simple vesicle (hb).
Medullary furrow still wide open from x; greatly widened at z. mp
medullary plates, sp lateral plates, y limit of gullet-cavity (sh) and
fore-gut (vd). Figure 1.154 with ten pairs of somites. Brain divided
into three vesicles: v fore-brain, m middle-brain, h hind-brain, c
heart, dv vitelline-veins. Medullary furrow still wide open behind
(z). mp medullary plates. Figure 1.155 with sixteen pairs of somites.
Brain divided into five vesicles: v fore-brain, z intermediate-brain,
m middle-brain, h hind-brain, n after-brain, a optic vesicles, g
auditory vesicles, c heart, dv vitelline veins, mp medullary plate, uw
primitive vertebra.)
It is, therefore, wrong to describe the first rudimentary segments in
the vertebrate embryo as primitive vertebrae or provertebrae; the fact
that they have been so called for some time has led to much error and
misunderstanding. Hence we shall give the name of "somites" or
primitive segments to these so-called "primitive vertebrae." If the
latter name is retained at all, it should only be used of the
sclerotom--i.e., the small part of the somites from which the later
vertebra does actually develop.
Articulation begins in all vertebrates at a very early embryonic
stage, and this indicates the considerable phylogenetic age of the
process. When the chordula (Figures 1.83 to 1.86) has completed its
characteristic composition, often even a little earlier, we find in
the amniotes, in the middle of the sole-shaped embryonic shield,
several pairs of dark square spots, symmetrically distributed on both
sides of the chorda (Figures 1.131 to 1.135). Transverse sections
(Figure 1.93 uw) show that they belong to the stem-zone (episoma) of
the mesoderm, and are separated from the parietal zone (hyposoma) by
the lateral folds; in section they are still quadrangular, almost
square, so that they look something like dice. These pairs of "cubes"
of the mesoderm are the first traces of the primitive segments or
somites, the so-called "protovertebrae." (Figures 1.153 to 1.155 uw).
(FIGURE 1.156. Embryo of the amphioxus, sixteen hours old, seen from
the back. (From Hatschek.) d primitive gut, u primitive mouth, p polar
cells of the mesoderm, c coelom-pouches, m their first segment, n
medullary tube, i entoderm, e ectoderm, s first segment-fold.
FIGURE 1.157. Embryo of the amphioxus, twenty hours old, with five
somites. (Right view; for left view see Figure 1.124.) (From
Hatschek.) V fore end, H hind end. ak, mk, ik outer, middle, and inner
germinal layers; dh alimentary canal, n neural tube, cn canalis
neurentericus, ush coelom-pouches (or primitive-segment cavities), us1
first (and foremost) primitive segment.)
Among the mammals the embryos of the marsupials have three pairs of
somites (Figure 1.131) after sixty hours, and eight pairs after
seventy-two hours (Figure 1.135). They develop more slowly in the
embryo of the rabbit; this has three somites on the eighth day (Figure
1.132), and eight somites a day later (Figure 1.134). In the incubated
hen's egg the first somites make their appearance thirty hours after
incubation begins (Figure 1.153). At the end of the second day the
number has risen to sixteen or eighteen (Figure 1.155). The
articulation of the stem-zone, to which the somites owe their origin,
thus proceeds briskly from front to rear, new transverse constrictions
of the "protovertebral plates" forming continuously and successively.
The first segment, which is almost half-way down in the embryonic
shield of the amniote, is the foremost of all; from this first somite
is formed the first cervical vertebra with its muscles and skeletal
parts. It follows from this, firstly, that the multiplication of the
primitive segments proceeds backwards from the front, with a constant
lengthening of the hinder end of the body; and, secondly, that at the
beginning of segmentation nearly the whole of the anterior half of the
sole-shaped embryonic shield of the amniote belongs to the later head,
while the whole of the rest of the body is formed from its hinder
half. We are reminded that in the amphioxus (and in our hypothetic
primitive vertebrate, Figures 1.98 to 1.102) nearly the whole of the
fore half corresponds to the head, and the hind half to the trunk.
The number of the metamera, and of the embryonic somites or primitive
segments from which they develop, varies considerably in the
vertebrates, according as the hind part of the body is short or is
lengthened by a tail. In the developed man the trunk (including the
rudimentary tail) consists of thirty-three metamera, the solid centre
of which is formed by that number of vertebrae in the vertebral column
(seven cervical, twelve dorsal, five lumbar, five sacral, and four
caudal). To these we must add at least nine head-vertebrae, which
originally (in all the craniota) constitute the skull. Thus the total
number of the primitive segments of the human body is raised to at
least forty-two; it would reach forty-five to forty-eight if
(according to recent investigations) the number of the original
segments of the skull is put at twelve to fifteen. In the tailless or
anthropoid apes the number of metamera is much the same as in man,
only differing by one or two; but it is much larger in the long-tailed
apes and most of the other mammals. In long serpents and fishes it
reaches several hundred (sometimes 400).
(FIGURES 1.158 TO 1.160. Embryo of the amphioxus, twenty four hours
old, with eight somites. (From Hatschek.) Figures 1.158 and 1.159
lateral view (from left). Figure 1.160 seen from back. In Figure 1.158
only the outlines of the eight primitive segments are indicated, in
Figure 1.159 their cavities and muscular walls. V fore end, H hind
end, d gut, du under and dd upper wall of the gut, ne canalis
neurentericus, nv ventral, nd dorsal wall of the neural tube, np
neuroporus, dv fore pouch of the gut, ch chorda, mf mesodermic fold,
pm polar cells of the mesoderm (ms), e ectoderm.)
In order to understand properly the real nature and origin of
articulation in the human body and that of the higher vertebrates, it
is necessary to compare it with that of the lower vertebrates, and
bear in mind always the genetic connection of all the members of the
stem. In this the simple development of the invaluable amphioxus once
more furnishes the key to the complex and cenogenetically modified
embryonic processes of the craniota. The articulation of the amphioxus
begins at an early stage--earlier than in the craniotes. The two
coelom-pouches have hardly grown out of the primitive gut (Figure
1.156 c) when the blind fore part of it (farthest away from the
primitive mouth, u) begins to separate by a transverse fold (s): this
is the first primitive segment. Immediately afterwards the hind part
of the coelom-pouches begins to divide into a series of pieces by new
transverse folds (Figure 1.157). The foremost of these primitive
segments (us1) is the first and oldest; in Figures 1.124 and 1.157
there are already five formed. They separate so rapidly, one behind
the other, that eight pairs are formed within twenty-four hours of the
beginning of development, and seventeen pairs twenty-four hours later.
The number increases as the embryo grows and extends backwards, and
new cells are formed constantly (at the primitive mouth) from the two
primitive mesodermic cells (Figures 1.159 to 1.160).
(FIGURES 1.161 AND 1.162. Transverse section of shark-embryos (through
the region of the kidneys). (From Wijhe and Hertwig.) In Figure 1.162
the dorsal segment-cavities (h) are already separated from the
body-cavity (lh), but they are connected a little earlier (Figure
1.161), nr neural tube, ch chorda, sch subchordal string, ao aorta, sk
skeletal-plate, mp muscle-plate, cp cutis-plate, w connection of
latter (growth-zone), vn primitive kidneys, ug prorenal duct, uk
prorenal canals, us point where they are cut off, tr prorenal funnel,
mk middle germ-layer (mk1 parietal, mk2 visceral), ik inner germ-layer
(gut-gland layer).)
This typical articulation of the two coelom-sacs begins very early in
the lancelet, before they are yet severed from the primitive gut, so
that at first each segment-cavity (us) still communicates by a narrow
opening with the gut, like an intestinal gland. But this opening soon
closes by complete severance, proceeding regularly backwards. The
closed segments then extend more, so that their upper half grows
upwards like a fold between the ectoderm (ak) and neural tube (n), and
the lower half between the ectoderm and alimentary canal (ch; Figure
1.82 d, left half of the figure). Afterwards the two halves completely
separate, a lateral longitudinal fold cutting between them (mk, right
half of Figure 1.82). The dorsal segments (sd) provide the muscles of
the trunk the whole length of the body (1.159): this cavity afterwards
disappears. On the other hand, the ventral parts give rise, from their
uppermost section, to the pronephridia or primitive-kidney canals, and
from the lower to the segmental rudiments of the sexual glands or
gonads. The partitions of the muscular dorsal pieces (myotomes)
remain, and determine the permanent articulation of the vertebrate
organism. But the partitions of the large ventral pieces (gonotomes)
become thinner, and afterwards disappear in part, so that their
cavities run together to form the metacoel, or the simple permanent
body-cavity.
The articulation proceeds in substantially the same way in the other
vertebrates, the craniota, starting from the coelom-pouches. But
whereas in the former case there is first a transverse division of the
coelom-sacs (by vertical folds) and then the dorso-ventral division,
the procedure is reversed in the craniota; in their case each of the
long coelom-pouches first divides into a dorsal (primitive segment
plates) and a ventral (lateral plates) section by a lateral
longitudinal fold. Only the former are then broken up into primitive
segments by the subsequent vertical folds; while the latter (segmented
for a time in the amphioxus) remain undivided, and, by the divergence
of their parietal and visceral plates, form a body-cavity that is
unified from the first. In this case, again, it is clear that we must
regard the features of the younger craniota as cenogenetically
modified processes that can be traced palingenetically to the older
acrania.
We have an interesting intermediate stage between the acrania and the
fishes in these and many other respects in the cyclostoma (the hag and
the lamprey, cf. Chapter 2.21).
(FIGURE 1.163. Frontal (or horizontal-longitudinal) section of a
triton-embryo with three pairs of primitive segments. ch chorda, us
primitive segments, ush their cavity, ak horn plate.)
Among the fishes the selachii, or primitive fishes, yield the most
important information on these and many other phylogenetic questions
(Figures 1.161 and 1.162). The careful studies of Ruckert, Van Wijhe,
H.E. Ziegler, and others, have given us most valuable results. The
products of the middle germinal layer are partly clear in these cases
at the period when the dorsal primitive segment cavities (or myocoels,
h) are still connected with the ventral body-cavity (lh; Figure
1.161). In Figure 1.162, a somewhat older embryo, these cavities are
separated. The outer or lateral wall of the dorsal segment yields the
cutis-plate (cp), the foundation of the connective corium. From its
inner or median wall are developed the muscle-plate (mp, the rudiment
of the trunk-muscles) and the skeletal plate, the formative matter of
the vertebral column (sk).
In the amphibia, also, especially the water-salamander (Triton), we
can observe very clearly the articulation of the coelom-pouches and
the rise of the primitive segments from their dorsal half (cf. Figure
1.91, A, B, C). A horizontal longitudinal section of the
salamander-embryo (Figure 1.163) shows very clearly the series of
pairs of these vesicular dorsal segments, which have been cut off on
each side from the ventral side-plates, and lie to the right and left
of the chorda.
(FIGURE 1.164. The third cervical vertebra (human).
FIGURE 1.165. The sixth dorsal vertebra (human).
FIGURE 1.166. The second lumbar vertebra (human).)
The metamerism of the amniotes agrees in all essential points with
that of the three lower classes of vertebrates we have considered; but
it varies considerably in detail, in consequence of cenogenetic
disturbances that are due in the first place (like the degeneration of
the coelom-pouches) to the large development of the food-yelk. As the
pressure of this seems to force the two middle layers together from
the start, and as the solid structure of the mesoderm apparently
belies the original hollow character of the sacs, the two sections of
the mesoderm, which are at that time divided by the lateral fold--the
dorsal segment-plates and ventral side-plates--have the appearance at
first of solid layers of cells (Figures 1.94 to 1.97). And when the
articulation of the somites begins in the sole-shaped embryonic
shield, and a couple of protovertebrae are developed in succession,
constantly increasing in number towards the rear, these cube-shaped
somites (formerly called protovertebrae, or primitive vertebrae) have
the appearance of solid dice, made up of mesodermic cells (Figure
1.93). Nevertheless, there is for a time a ventral cavity, or
provertebral cavity, even in these solid "protovertebrae" (Figure
1.143 uwh). This vesicular condition of the provertebra is of the
greatest phylogenetic interest; we must, according to the coelom
theory, regard it as an hereditary reproduction of the hollow dorsal
somites of the amphioxus (Figures 1.156 to 1.160) and the lower
vertebrates (Figures 1.161 to 1.163). This rudimentary "provertebral
cavity" has no physiological significance whatever in the
amniote-embryo; it soon disappears, being filled up with cells of the
muscular plate.
(FIGURE 1.167. Head of a shark embryo (Pristiurus), one-third of an
inch long, magnified twenty times. (From Parker.) Seen from the
ventral side.)
The innermost median part of the primitive segment plates, which lies
immediately on the chorda (Figure 1.145 ch) and the medullary tube
(m), forms the vertebral column in all the higher vertebrates (it is
wanting in the lowest); hence it may be called the skeleton plate. In
each of the provertebrae it is called the "sclerotome" (in opposition
to the outlying muscular plate, the "myotome"). From the phylogenetic
point of view the myotomes are much older than the sclerotomes. The
lower or ventral part of each sclerotome (the inner and lower edge of
the cube-shaped provertebra) divides into two plates, which grow round
the chorda, and thus form the foundation of the body of the vertebra
(wh). The upper plate presses between the chorda and the medullary
tube, the lower between the chorda and the alimentary canal (Figure
1.137 C). As the plates of two opposite provertebral pieces unite from
the right and left, a circular sheath is formed round this part of the
chorda. From this develops the BODY of a vertebra--that is to say, the
massive lower or ventral half of the bony ring, which is called the
"vertebra" proper and surrounds the medullary tube (Figures 1.164 to
1.166). The upper or dorsal half of this bony ring, the vertebral arch
(Figure 1.145 wb), arises in just the same way from the upper part of
the skeletal plate, and therefore from the inner and upper edge of the
cube-shaped primitive vertebra. As the upper edges of two opposing
somites grow together over the medullary tube from right and left, the
vertebra-arch becomes closed.
The whole of the secondary vertebra, which is thus formed from the
union of the skeletal plates of two provertebral pieces and encloses a
part of the chorda in its body, consists at first of a rather soft
mass of cells; this afterwards passes into a firmer, cartilaginous
stage, and finally into a third, permanent, bony stage. These three
stages can generally be distinguished in the greater part of the
skeleton of the higher vertebrates; at first most parts of the
skeleton are soft, tender, and membranous; they then become
cartilaginous in the course of their development, and finally bony.
(FIGURES 1.168 AND 1.169. Head of a chick embryo, of the third day.
Figure 1.168 from the front, Figure 1.169 from the right. n
rudimentary nose (olfactory pit), l rudimentary eye (optic pit,
lens-cavity), g rudimentary ear (auditory pit), v fore-brain, gl
eye-cleft. Of the three pairs of gill-arches the first has passed into
a process of the upper jaw (o) and of the lower jaw (u). (From
Kolliker.))
At the head part of the embryo in the amniotes there is not generally
a cleavage of the middle germinal layer into provertebral and lateral
plates, but the dorsal and ventral somites are blended from the first,
and form what are called the "head-plates" (Figure 1.148 k). From
these are formed the skull, the bony case of the brain, and the
muscles and corium of the body. The skull develops in the same way as
the membranous vertebral column. The right and left halves of the head
curve over the cerebral vesicle, enclose the foremost part of the
chorda below, and thus finally form a simple, soft, membranous capsule
about the brain. This is afterwards converted into a cartilaginous
primitive skull, such as we find permanently in many of the fishes.
Much later this cartilaginous skull becomes the permanent bony skull
with its various parts. The bony skull in man and all the other
amniotes is more highly differentiated and modified than that of the
lower vertebrates, the amphibia and fishes. But as the one has arisen
phylogenetically from the other, we must assume that in the former no
less than the latter the skull was originally formed from the
sclerotomes of a number of (at least nine) head-somites.
While the articulation of the vertebrate body is always obvious in the
episoma or dorsal body, and is clearly expressed in the segmentation
of the muscular plates and vertebrae, it is more latent in the
hyposoma or ventral body. Nevertheless, the hyposomites of the vegetal
half of the body are not less important than the episomites of the
animal half. The segmentation in the ventral cavity affects the
following principal systems of organs: 1, the gonads or sex-glands
(gonotomes); 2, the nephridia or kidneys (nephrotomes); and 3, the
head-gut with its gill-clefts (branchiotomes).
(FIGURE 1.170. Head of a dog embryo, seen from the front. a the two
lateral halves of the foremost cerebral vesicle, b rudimentary eye, c
middle cerebral vesicle, de first pair of gill-arches (e upper-jaw
process, d lower-jaw process), f, f apostrophe, f double apostrophe,
second, third, and fourth pairs of gill-arches, g h i k heart (g
right, h left auricle; i left, k right ventricle), l origin of the
aorta with three pairs of arches, which go to the gill-arches. (From
Bischoff.))
The metamerism of the hyposoma is less conspicuous because in all the
craniotes the cavities of the ventral segments, in the walls of which
the sexual products are developed, have long since coalesced, and
formed a single large body-cavity, owing to the disappearance of the
partition. This cenogenetic process is so old that the cavity seems to
be unsegmented from the first in all the craniotes, and the rudiment
of the gonads also is almost always unsegmented. It is the more
interesting to learn that, according to the important discovery of
Ruckert, this sexual structure is at first segmental even in the
actual selachii, and the several gonotomes only blend into a simple
sexual gland on either side secondarily.
(FIGURE 1.171. Human embryo of the fourth week (twenty-six days old),
one-fourth of an inch in length magnified twenty times, showing: point
of development of the hind-leg, umbilical cord (underneath it the
tail, bent upwards), trigeminal nerve V Trigeminus, optic-muscle nerve
III Oculo-motorius, rolling muscle nerve IV Trochlearis, rudiment of
ear (labyrinthic vesicles), pneumogastric nerve X Vagus, terminal
nerve XI Accessorius, hypoglossal nerve XII Hypoglossus, first spinal
nerve, point of development of arm (or fore-leg), true spinal nerve.
(From Moll.) The rudiments of the cerebral nerves and the roots of the
spinal nerves are especially marked. Underneath the four gill-arches
(left side) is the heart (with auricle, V and ventricle, K), under
this again the liver (L).)
Amphioxus, the sole surviving representative of the acrania, once more
yields us most interesting information; in this case the sexual glands
remain segmented throughout life. The sexually mature lancelet has, on
the right and left of the gut, a series of metamerous sacs, which are
filled with ova in the female and sperm in the male. These segmental
gonads are originally nothing else than the real gonotomes, separate
body-cavities, formed from the hyposomites of the trunk.
The gonads are the most important segmental organs of the hyposoma, in
the sense that they are phylogenetically the oldest. We find sexual
glands (as pouch-like appendages of the gastro-canal system) in most
of the lower animals, even in the medusae, etc., which have no
kidneys. The latter appear first (as a pair of excretory tubes) in the
platodes (turbellaria), and have probably been inherited from these by
the articulates (annelids) on the one hand and the unarticulated
prochordonia on the other, and from these passed to the articulated
vertebrates. The oldest form of the kidney system in this stem are the
segmental pronephridia or prorenal canals, in the same arrangement as
Boveri found them in the amphioxus. They are small canals that lie in
the frontal plane, on each side of the chorda, between the episoma and
hyposoma (Figure 1.102 n); their internal funnel-shaped opening leads
into the various body-cavities, their outer opening is the lateral
furrow of the epidermis. Originally they must have had a double
function, the carrying away of the urine from the episomites and the
release of the sexual cells from the hyposomites.
The recent investigations of Ruckert and Van Wijhe on the mesodermic
segments of the trunk and the excretory system of the selachii show
that these "primitive fishes" are closely related to the amphioxus in
this further respect. The transverse section of the shark-embryo in
Figure 1.161 shows this very clearly.
In other higher vertebrates, also, the kidneys develop (though very
differently formed later on) from similar structures, which have been
secondarily derived from the segmental pronephridia of the acrania.
The parts of the mesoderm at which the first traces of them are found
are usually called the middle or mesenteric plates. As the first
traces of the gonads make their appearance in the lining of these
middle plates nearer inward (or the middle) from the inner funnels of
the nephro-canals, it is better to count this part of the mesoderm
with the hyposoma.
The chief and oldest organ of the vertebrate hyposoma, the alimentary
canal, is generally described as an unsegmented organ. But we could
just as well say that it is the oldest of all the segmented organs of
the vertebrate; the double row of the coelom-pouches grows out of the
dorsal wall of the gut, on either side of the chorda. In the brief
period during which these segmental coelom-pouches are still openly
connected with the gut, they look just like a double chain of
segmented visceral glands. But apart from this, we have originally in
all vertebrates an important articulation of the fore-gut, that is
wanting in the lower gut, the segmentation of the branchial (gill)
gut.
(FIGURE 1.172. Transverse section of the shoulder and fore-limb (wing)
of a chick-embryo of the fourth day, magnified about twenty times.
Beside the medullary tube we can see on each side three clear streaks
in the dark dorsal wall, which advance into the rudimentary fore-limb
or wing (e). The uppermost of them is the muscular plate; the middle
is the hind and the lowest the fore root of a spinal nerve. Under the
chorda in the middle is the single aorta, at each side of it a
cardinal vein, and below these the primitive kidneys. The gut is
almost closed. The ventral wall advances into the amnion, which
encloses the embryo. (From Remak.)
FIGURE 1.173. Transverse section of the pelvic region and hind legs of
a chick-embryo of the fourth day, magnified about forty times. h
horn-plate, w medullary tube, n canal of the tube, u primitive
kidneys, x chorda, e hind legs, b allantoic canal in the ventral wall,
t aorta, v cardinal veins, a gut, d gut-gland layer, f gut-fibre
layer, g embryonic epithelium, r dorsal muscles, c body-cavity or
coeloma. (From Waldeyer.))
The gill-clefts, which originally in the older acrania pierced the
wall of the fore-gut, and the gill-arches that separated them, were
presumably also segmental, and distributed among the various metamera
of the chain, like the gonads in the after-gut and the nephridia. In
the amphioxus, too, they are still segmentally formed. Probably there
was a division of labour of the hyposomites in the older (and long
extinct) acrania, in such wise that those of the fore-gut took over
the function of breathing and those of the after-gut that of
reproduction. The former developed into gill-pouches, the latter into
sex-pouches. There may have been primitive kidneys in both. Though the
gills have lost their function in the higher animals, certain parts of
them have been generally maintained in the embryo by a tenacious
heredity. At a very early stage we notice in the embryo of man and the
other amniotes, at each side of the head, the remarkable and important
structures which we call the gill-arches and gill-clefts (Figures
1.167 to 1.170 f). They belong to the characteristic and inalienable
organs of the amniote-embryo, and are found always in the same spot
and with the same arrangement and structure. There are formed to the
right and left in the lateral wall of the fore-gut cavity, in its
foremost part, first a pair and then several pairs of sac-shaped
inlets, that pierce the whole thickness of the lateral wall of the
head. They are thus converted into clefts, through which one can
penetrate freely from without into the gullet. The wall thickens
between these branchial folds, and changes into an arch-like or
sickle-shaped piece--the gill, or gullet-arch. In this the muscles and
skeletal parts of the branchial gut separate; a blood-vessel arch
rises afterwards on their inner side (Figure 1.98 ka). The number of
the branchial arches and the clefts that alternate with them is four
or five on each side in the higher vertebrates (Figure 1.170 d, f, f
apostrophe, f double apostrophe). In some of the fishes (selachii) and
in the cyclostoma we find six or seven of them permanently.
These remarkable structures had originally the function of respiratory
organs--gills. In the fishes the water that serves for breathing, and
is taken in at the mouth, still always passes out by the branchial
clefts at the sides of the gullet. In the higher vertebrates they
afterwards disappear. The branchial arches are converted partly into
the jaws, partly into the bones of the tongue and the ear. From the
first gill-cleft is formed the tympanic cavity of the ear.
There are few parts of the vertebrate organism that, like the outer
covering or integument of the body, are not subject to metamerism. The
outer skin (epidermis) is unsegmented from the first, and proceeds
from the continuous horny plate. Moreover, the underlying cutis is
also not metamerous, although it develops from the segmental structure
of the cutis-plates (Figures 1.161 and 1.162 cp). The vertebrates are
strikingly and profoundly different from the articulates in these
respects also.
Further, most of the vertebrates still have a number of unarticulated
organs, which have arisen locally, by adaptation of particular parts
of the body to certain special functions. Of this character are the
sense-organs in the episoma, and the limbs, the heart, the spleen, and
the large visceral glands--lungs, liver, pancreas, etc.--in the
hyposoma. The heart is originally only a local spindle-shaped
enlargement of the large ventral blood-vessel or principal vein, at
the point where the subintestinal passes into the branchial artery, at
the limit of the head and trunk (Figures 1.170 and 1.171). The three
higher sense-organs--nose, eye, and ear--were originally developed in
the same form in all the craniotes, as three pairs of small
depressions in the skin at the side of the head.
The organ of smell, the nose, has the appearance of a pair of small
pits above the mouth-aperture, in front of the head (Figure 1.169 n).
The organ of sight, the eye, is found at the side of the head, also in
the shape of a depression (Figures 1.169 l and 1.170 b), to which
corresponds a large outgrowth of the foremost cerebral vesicle on each
side. Farther behind, at each side of the head, there is a third
depression, the first trace of the organ of hearing (Figure 1.169 g).
As yet we can see nothing of the later elaborate structure of these
organs, nor of the characteristic build of the face.
(FIGURE 1.174. Development of the lizard's legs (Lacerta agilis), with
special relation to their blood-vessels. 1, 3, 5, 7, 9, 11 right
fore-leg; 13, 15 left fore-leg; 2, 4, 6, 8, 10, 12 right hind-leg; 14,
16 left hind-leg; SRV lateral veins of the trunk, VU umbilical vein.
(From F. Hochstetter.))
When the human embryo has reached this stage of development, it can
still scarcely be distinguished from that of any other higher
vertebrate. All the chief parts of the body are now laid down: the
head with the primitive skull, the rudiments of the three higher
sense-organs and the five cerebral vesicles, and the gill-arches and
clefts; the trunk with the spinal cord, the rudiment of the vertebral
column, the chain of metamera, the heart and chief blood-vessels, and
the kidneys. At this stage man is a higher vertebrate, but shows no
essential morphological difference from the embryos of the mammals,
the birds, the reptiles, etc. This is an ontogenetic fact of the
utmost significance. From it we can gather the most important
phylogenetic conclusions.
There is still no trace of the limbs. Although head and trunk are
separated and all the principal internal organs are laid down, there
is no indication whatever of the "extremities" at this stage; they are
formed later on. Here again we have a fact of the utmost interest. It
proves that the older vertebrates had no feet, as we find to be the
case in the lowest living vertebrates (amphioxus and the cyclostoma).
The descendants of these ancient footless vertebrates only acquired
extremities--two fore-legs and two hind-legs--at a much later stage of
development. These were at first all alike, though they afterwards
vary considerably in structure--becoming fins (of breast and belly) in
the fishes, wings and legs in the birds, fore and hind legs in the
creeping animals, arms and legs in the apes and man. All these parts
develop from the same simple original structure, which forms
secondarily from the trunk-wall (Figures 1.172 and 1.173). They have
always the appearance of two pairs of small buds, which represent at
first simple roundish knobs or plates. Gradually each of these plates
becomes a large projection, in which we can distinguish a small inner
part and a broader outer part. The latter is the rudiment of the foot
or hand, the former that of the leg or arm. The similarity of the
original rudiment of the limbs in different groups of vertebrates is
very striking.
(FIGURE 1.175. Human embryo, five weeks old, half an inch long, seen
from the right, magnified ten times. (From Russel Bardeen and Harmon
Lewis.) In the undissected head we see the eye, mouth, and ear. In the
trunk the skin and part of the muscles have been removed, so that the
cartilaginous vertebral column is free; the dorsal root of a spinal
nerve goes out from each vertebra (towards the skin of the back). In
the middle of the lower half of the figure part of the ribs and
intercostal muscles are visible. The skin and muscles have also been
removed from the right limbs; the internal rudiments of the five
fingers of the hand, and five toes of the foot, are clearly seen
within the fin-shaped plate, and also the strong network of nerves
that goes from the spinal cord to the extremities. The tail projects
under the foot, and to the right of it is the first part of the
umbilical cord.)
How the five fingers or toes with their blood-vessels gradually
differentiate within the simple fin-like structure of the limbs can be
seen in the instance of the lizard in Figure 1.174. They are formed in
just the same way in man: in the human embryo of five weeks the five
fingers can clearly be distinguished within the fin-plate (Figure
1.175).
The careful study and comparison of human embryos with those of other
vertebrates at this stage of development is very instructive, and
reveals more mysteries to the impartial student than all the religions
in the world put together. For instance, if we compare attentively the
three successive stages of development that are represented, in twenty
different amniotes we find a remarkable likeness. When we see that as
a fact twenty different amniotes of such divergent characters develop
from the same embryonic form, we can easily understand that they may
all descend from a common ancestor.
(FIGURES 1.176 TO 1.178. Embryos of the bat (Vespertilio murinus) at
three different stages. (From Oscar Schultze.) Figure 1.176:
Rudimentary limbs (v fore-leg, h hind-leg). l lenticular depression, r
olfactory pit, ok upper jaw, uk lower jaw, k2, k3, k4 first, second
and third gill-arches, a amnion, n umbilical vessel, d yelk-sac.
Figure 1.177: Rudiment of flying membrane, membranous fold between
fore and hind leg. n umbilical vessel, o ear-opening, f flying
membrane. Figure 1.178: The flying membrane developed and stretched
across the fingers of the hands, which cover the face.)
In the first stage of development, in which the head with the five
cerebral vesicles is already clearly indicated, but there are no
limbs, the embryos of all the vertebrates, from the fish to man, are
only incidentally or not at all different from each other. In the
second stage, which shows the limbs, we begin to see differences
between the embryos of the lower and higher vertebrates; but the human
embryo is still hardly distinguishable from that of the higher
mammals. In the third stage, in which the gill-arches have disappeared
and the face is formed, the differences become more pronounced. These
are facts of a significance that cannot be exaggerated.* (* Because
they show how the most diverse structures may be developed from a
common form. As we actually see this in the case of the embryos, we
have a right to assume it in that of the stem-forms. Nevertheless,
this resemblance, however great, is never a real identity. Even the
embryos of the different individuals of one species are usually not
really identical. If the reader can consult the complete edition of
this work at a library, he will find six plates illustrating these
twenty embryos.)
If there is an intimate causal connection between the processes of
embryology and stem-history, as we must assume in virtue of the laws
of heredity, several important phylogenetic conclusions follow at once
from these ontogenetic facts. The profound and remarkable similarity
in the embryonic development of man and the other vertebrates can only
be explained when we admit their descent from a common ancestor. As a
fact, this common descent is now accepted by all competent scientists;
they have substituted the natural evolution for the supernatural
creation of organisms.